Unable to access both partitions on external hard drive

Hi. I use a 1tb external hard drive to store music, photos and movies. I use it on my Imac, also on my Asus e-book. I see that the external hard drive is partitioned into two. How or why I don't know. What I do know is that the first part is now full, and I cant see the second part when plugged into the USB on either computer. Could anyone help me by suggesting how I can access the second 500gig? Id appreciate answers in plain English, I really dont understand more than the basics, and the clearer ans simpler the answers the more I will learn, and thanks to all those about to read this !